The present invention relates to a pit prop. More particularly this invention concerns such a pit prop useable in a mine at the face and connected to the face conveyor for supporting the mine roof immediately adjacent the face.
It is known to connect a plurality of pit props to a face conveyor. Each of these pit props comprises a foot that engages the floor and is shaped as a skid, an arm pivoted at the rear end of the skid and extending forwardly toward the face, a shield carried at the front end of the arm and engageable with the mine roof, and a hydraulic ram between the foot and the arm for pressing the shield against the roof. Such an assembly serves to prevent cave-ins at the face.
Typically each of these pit props is connected via a hydraulic ram to the face conveyor for displacement of the pit prop and of the face conveyor by means of these rams. Thus when all of the pit props are locked between the mine floor and ceiling it is possible by actuation of the rams to displace the face conveyor toward or away from the face. Similarly, when all but one of the pit props are locked between the mine roof and ceiling, it is possible to relax the main ram of one of the props and use its displacement ram connected to the face conveyor to displace this prop relative to the conveyor. In this manner it is possible to move these pit props and the face conveyor along as the seam is worked.
A considerable disadvantage of such systems is that the foot of the pit prop is often pressed into the ground by the considerable force exerted by its main ram pressing the shield against the roof. In order subsequently to displace the pit prop it is necessary either somehow to lift the front end at least of the foot out of the hole formed by it, or to pull it with such a force that it can simply push aside the material in which it has embedded itself.
It has been suggested to provide both at the front and rear ends of the floor-engaging foot or skid of the pit prop a vertical ram connected at its upper end to the foot and at its lower end carrying a floor-engageable skid. Thus if either of these ends sinks into the floor it is possible to operate this vertical ram and lift up the sinking end. Such an arrangement has the difficulty that the frequently soft floor does not provide a sufficiently sound purchase so the extra lifting skid merely will sink in also. Furthermore, such extra mechanism requires considerable redesigning of the pit prop, as the space for such a mechanism, at least at the front end of the prop, is normally occupied by other structure.